In recent years, a light-emitting device in combination of a light-emitting element emitting primary light and a wavelength conversion unit absorbing the primary light and emitting secondary light has quickly become popular as a light-emitting device that has a little power consumption, that is small, and that is able to realize a color reproduction over a wide range with high brightness. Research and development of such a light-emitting device are actively ongoing with intent to improve the device performance.
Here, light in a range from an ultraviolet ray having a relatively long wavelength to blue light is usually employed as the primary light emitted from the light-emitting element. Furthermore, while various types of fluorescent materials suitable for use in light-emitting devices are optionally used in the wavelength conversion unit, fluorescent materials in the form of oxides, for example, are employed in many cases.
Under such a situation, oxynitride fluorescent materials have more recently been proposed which are nitride fluorescent materials capable of being exited by excitation light that is emitted from a semiconductor light-emitting device using a nitride semiconductor, such as GaN. The oxynitride fluorescent materials are thermally and chemically more stable than known fluorescent materials, and exhibit high absorption performance for light having wavelengths from a near-ultraviolet range to a visible range.
Patent Literature (PTL) 1 discloses that, among the oxynitride fluorescent materials, particularly a β SiAlON fluorescent material provides a light emission spectrum in a sharp profile with a peak located in a wavelength range of 500 nm to 550 nm, and takes (x, y) values in ranges of 0≦x≦0.3 and 0.6≦y≦0.83 on the CIE coordinates. PTL 1 further states that the β SiAlON fluorescent material is used in lighting apparatuses and image displays.
PTL 2 states that light emission intensity of a Eu-activated β SiAlON fluorescent material is increased by setting a 50%-area average diameter of primary particles of the Eu-activated β SiAlON fluorescent material to be 5 μm or more, light emission intensity of a light-emitting device using the Eu-activated β SiAlON fluorescent material is increased.